A driver should take the most efficient route to get you to your destination on time. If the ride took longer than expected, but there was a lot of traffic, it probably wasn’t the driver’s fault. However, keep an eye out for drivers who intentionally drive slowly or take detours to hike up the fare. Review the quality of the ride. Weigh factors like the driver’s knowledge of the area, the speed of his service, his extra effort, and his general demeanor. These indices are absolutely subjective – so follow your gut and adjust your tips on a case-by-case basis.
Follow these general guidelines for tipping based on quality. Tip 5-10% (or not at all) if the service was particularly poor. Perhaps the driver took the longest route to get you to your destination; perhaps he was rude to you, or made you feel uncomfortable. Tip 15% for a solid, middle-of-the-road cab ride. The driver brought you to your destination in a reasonable amount of time and made you feel comfortable, but didn’t do anything exceptional. 15% is acceptable in most cases. Tip 20% or higher if this was one of the better cab rides you’ve ever had. Maybe you had a great conversation with the cabbie, and s/he gave you some interesting ideas for exploring the area; maybe s/he took a lesser-known back-road that got you to the airport much faster than if you’d sat through traffic; maybe s/he helped you haul your bags to the doorstep.